Conventionally, carbon fibers are produced by forming a starting material such as pitch or polyacrylonitrile into fibers while maintaining the main chain framework of the starting material. This type of method is incapable of producing a product wherein the nano-scale molecules are controlled.
Carbon nanotubes (hereinafter referred to as “CNTs”), which have been attracting attention recently, can be roughly regarded as fibers controlled at the molecular level. CNTs are produced by the carbon arc method, sputtering, laser beam irradiation or like technique, using graphite or like starting carbon material in the presence of a metal catalyst. However, these techniques chiefly produce carbon nanotubes having a graphitic structure. One example of a carbon nanotube of such a structure is a carbon filament comprising a graphitic core surrounded by irregular pyrrolytic carbon (Oberlin, Endo, Koyama; Carbon 14, 133 (1976)).
Also, Japanese Examined Patent Publications No. 1991-64606 and No. 1991-77288 and other publications disclose carbon fibrils each comprising a graphitic outer portion and an inner core comprising irregular carbon atoms. In the disclosed techniques, however, it is practically difficult to control straightness of molecules or shape factors such as diameter and length.
Further, attempts have been made to produce CNTs using acetylene or like gaseous hydrocarbon as a starting material in the presence of a catalyst such as iron. However, also in these techniques, it is practically difficult to control straightness of molecules, diameter, length and the like while maintaining the graphitic structure. Stated specifically, in all of these techniques, the starting materials are excited into a gas-phase activated carbon state and then allowed to form CNTs during the process of recombination. Therefore, it is extremely difficult to control the reaction parameters such as the amount of the starting material, resulting in production of CNTs having a graphitic structure and varying widely in shape. The products obtained by these techniques have a degree of graphitization (crystallinity) of at least 5%, and 50% to 100% in most cases. Further, the end each of the CNTs is closed with a cap and CNTs often contain a metal at the tips.
One report points out the existence of carbon nanotubes having an amorphous structure, as a precursor of graphitic carbon nanotubes. However, the existence of the carbon nanotubes with an amorphous structure is merely presumed from the existence of carbon tubes with an amorphous structure which are observed, through TEM, among the graphitic carbon nanotubes. Moreover, the amorphous carbon tubes are reported to be an intermediate product temporarily formed in the process of forming the graphitic carbon tubes. Thus, the selective synthesis method or use of the amorphous carbon nanotubes have not been elucidated yet (Wenlow Wang et al: Electrochemical Society Proceedings Volume 97-14, 814(1997)).
As discussed above, it is practically difficult for the conventional techniques to control the crystal structure, molecular straightness, diameter, length, end structure and the like of the CNTs. In particular, the CNTs have a substantially graphitic structure, and thus have low degree of freedom of structural control. Moreover, the conventional techniques have the problem that amorphous carbon formed as a by-product contaminates the graphite product and makes purification extremely difficult.
Regarding the properties of CNTs, it has been reported that CNTs are likely to adsorb a hydrogen gas densely by their capillary action (A. C. Dillon et al: Nature, 386, 377(1997)). Further, U.S. Pat. No. 5,653,951 states that solid layered nanostructures (graphite nanofibers) are capable of chemisorbing a large amount of hydrogen into the interstices of the graphite layers.
However, reports on these known materials merely point out possible performance characteristics of the materials on a research and development level. This is because the prior art techniques have a number of problems such as difficulties in material synthesis process and in control of structure and shape that affect the material stability, lack of mass-productivity, and the like. Moreover, the graphite nanofibers do not have sufficient durability for repetitive use, since the distance between the graphite layers expands as the fibers adsorb hydrogen. Accordingly, there remain a number of problems to be solved before putting the known materials into practical use.